The present invention relates to a multi-carrier communication apparatus, a power line communication circuit, and a multi-carrier communication method which perform communication using a plurality of carriers.
A transmission method using a plurality of sub-carriers such as an orthogonal frequency division multiplexing (OFDM) method has a big advantage that a high quality of communication is possible even on an extremely poor conditioned transmission line. Accordingly, it is used in the wired communication such as a power line communication as well as the wireless communication.
In a multi-carrier communication apparatus using the plurality of sub-carriers to perform communication, a transmitter converts bit data which are scheduled to be transmitted into symbol data, performs symbol mapping in accordance with the symbol data, converts them into data on a time domain by performing the inverse fast Fourier transform (FFT) or inverse wavelet transform, and converts them into a base-band analog signal by performing the serial-parallel conversion and DA conversion, and then sends the converted signal. Conversely, a receiver converts the received signal into digital signal by performing the AD conversion, converts them into data on a frequency domain by performing the serial-parallel conversion and then the FFT conversion or wavelet conversion, and obtains received bit data by performing demapping.
In this manner, the multi-carrier communication apparatus has a carrier-detecting function to determine whether other apparatuses are transmitting signal to the transmission line or not. Accordingly, when other apparatus do not use the transmission line, the multi-carrier communication apparatus performs a transmitting process on demand.
The carrier detection of the multi-carrier communication apparatus, as disclosed in US 2005-0037722A1, for example, is performed on the basis of a signal after digital waveform data converted by means of the AD converter are converted into data on the frequency domain.
In such a carrier detection method based on the signal having been converted into the data on the frequency domain, since a correlation between the adjacent sub-carriers is computed on the frequency domain by taking advantage of the OFDM's characteristics that the plurality of sub-carriers are arranged at a constant interval, it is possible to detection the carrier with a high precision.
However, the carrier detection method can detect the carrier when a signal is transmitted from one communication apparatus on the transmission line or when signals are transmitted from two communication apparatuses or more at the same time without time discord. However, when the signals are transmitted from two communication apparatuses or more at the same time at the state of the time discord, there is a case that the carrier cannot be detected in spite of the fact that the signals exist. Accordingly, the communication apparatuses which cannot detect the carrier judge that the signals do not exist and transmit signals to the transmission line. As a result, when collision of the transmitted signals happens in series, there is an occasion when communication is impossible.
When signals are transmitted from two communication apparatuses on the transmission line at the same time without the time discord in FIG. 24, there are frequency spectrums of all sub-carriers (non-zero) in FIG. 25. Accordingly, correlation between the adjacent sub-carriers can be taken. On the other hand, when the signals are transmitted at the same time at the state of the time discord by an OFDM symbol period (Ts) in FIG. 26, levels of the sub-carriers become 0 at every other interval in FIG. 27. Accordingly, the correlation between the adjacent sub-carriers cannot be taken. That is, it cannot be recognized that a desired carrier exists on the transmission line